This invention relates to a throttle control for an aircraft turbofan engine, and more particularly to a throttle control, which eliminates dead band in the throttle control.
Typically, two different power setting parameters are used to control the speed of operation of a turbofan engine. These are fan speed at high power setting and engine core speed at idle. Historically, the transition to fan speed from core speed causes a dead band or slippage in the throttle response. The dead band is different for each engine due to the variation in engine hardware and control sensors.
The disclosure in U.S. Pat. No. 4,296,601 seeks to address this problem by controlling a combined engine speed parameter. The combined speed parameter is comprised of core speed and fan speed. The combined parameter is correlated with power lever or throttle position so as to control fuel flow to the engine at variant power level requirements.
As indicated, many turbofan engines with electronic engine controls use fan speed as the power setting parameter for high power operation. Climb and takeoff power settings are examples of high power operation. However, at idle the same engine may use core (also known as HP shaft) speed as the power setting parameter. Typically, an equivalent idle power set fan speed is estimated for the core idle speed. The idle fan speed and the climb power setting fan provide end points for the engine throttle.
There is variability in the engine control sensors and from one engine""s hardware to the next. Consequently, the estimated speed for idle may only be representative of a small number of engines. When operating at the estimated fan speed for idle some engines may have a core speed higher than the power setting core speed. To ensure all engines obtain the power setting core speed, the power set fan speed for idle is lowered below the estimated value. The lowering of the power set fan speed will ensure the engine transitions on to the core speed idle governor. However, this approach to transition from fan speed to core speed power setting often causes dead bands in the engine throttle movement.
The size of the dead band will vary depending on the individual engine characteristics. Consequently, on multi-engined aircraft the throttle for each engine will likely have a different dead band and could change as engines are replaced as part of normal maintenance.
As can be seen, there is a need for an apparatus and method to provide smooth transition from fan speed to core speed control, eliminating the throttle dead band. An apparatus and method is also needed that is stable dynamically and always ensures a flat or increased fan speed with increasing throttle position A further need is for engine throttle position or movement to result in a commanded fuel flow. Also needed is an apparatus and method that varies fuel flow to obtain a desired fan speed or idle core speed or set points there between to eliminate the dead band. Yet another need is for an apparatus and method that transitions from one controlling parameter to an entirely different controlling parameter and performs a selection between the various parameters to define the final control parameter to attack the direct cause of dead bands in throttle control of turbofan engines, such as by monitoring fuel flow to the engine to eliminate any dead band intervals
In accordance with the present invention, a turbofan engine control system and method for eliminating dead bands in the throttle control decreases and increases the fuel flow to the engine during transition from the scheduled core speed to the scheduled fan speed of the engine to effect a smooth and continuous transition from the core speed to the fan speed and vice-versa. This is effected by generating a signal to a control unit to open and close a valve system to augment and/or decrease the fuel flow in response to sensing different throttle lever angles of the throttle in the control system.
In one aspect of the present invention a throttle lever for controlling engine speed is provided along with a fuel section for providing fuel flow to the engine at different throttle lever angles in proportion to the throttle lever angle, with the fuel section including a control mechanism for augmenting and decreasing the fuel flow in response to a fuel flow modifying electrical signal and a signal processing circuit for providing the fuel flow modifying signal in response to sensing different throttle lever angles representing a combination of the fan and core idle speed scheduling.
In another aspect of the invention, the signal processing circuit provides a signal which achieves a smooth and continuous transition from the core speed to the fan speed scheduling to eliminate any dead bands in the throttle lever angle.
In yet another aspect of the invention, a plurality of turbofan engines can be controlled by the system of the invention by providing a separate throttle lever for controlling engine speed of each engine, a fuel section, and a signal processing circuit for providing a fuel flow modifying electrical signal to the fuel section in response to sensing the different throttle lever angles associated with each engine representative of the combination of fan and core idle speed scheduling of each engine.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.